1. Field of the Invention
This invention relates to an image forming apparatus and more particularly to an image forming apparatus comprising an optical scanner for deflecting light beams emitted from a plurality of light sources by a single deflector (for example, a rotating polygon mirror), dividing one scanning line on an image carrier (for example, a photosensitive drum) whose surface moves in a predetermined direction by the deflected light beams, and scanning the light beams over the line in a main scanning direction.
2. Description of the Related Art
Hitherto, an optical scanner for emitting light beams from two light sources, deflecting the two emitted light beams by a polygon mirror, dividing one scanning line on a photosensitive body by the two deflected light beams, and scanning the light beams over the line has been proposed (refer to Japanese Patent Unexamined Publication No. Sho. 63-47718).
The optical scanner scans one light beam over the first half of an image area of the photosensitive body and the other beam over the latter half of the image area of the photosensitive body. The scanning line is thus divided by two light beams and the light beams are scanned, whereby the number of images of the polygon mirror can be increased and the print speed can be improved without upsizing the optical scanner.
By the way, to divide one scanning line by two light beams and scan the light beams, if an image formed by scanning one light beam is assumed to be one panel image, images formed by scanning two light beams are formed as a set of panel images.
The light beams are generated from light sources placed at different positions and arrive at the photosensitive body over different passages. Thus, the placement positions of the light sources, the positions of the parts making up the light sources, the positions of optical systems placed on the passages, and the like are displaced due to external causes such as temperature change, vibration, and shock, and the light beam passage may shift from a predetermined passage.
If the light beam passage thus shifts, a discontinuous image occurs in the joint between panel images formed by scanning the light beams and it is feared that the image quality may be degraded remarkably.
The images formed from light sources on both sides of the joint according to image signals having the same number of clocks may differ in length (namely, a partial magnification difference may occur), and to form a multicolor image, it is feared that a color misregistration may be caused.